xen/p2m: Fix p2m_flush_table for non-nested cases

xen/p2m: Fix p2m_flush_table for non-nested cases

Commit 71bb7304e7a7a35ea6df4b0cedebc35028e4c159 added flushing of
nested p2m tables whenever the host p2m table changed.  Unfortunately
in the process, it added a filter to p2m_flush_table() function so
that the p2m would only be flushed if it was being used as a nested
p2m.  This meant that the p2m was not being flushed at all for altp2m
callers.

Only check np2m_base if p2m_class for nested p2m's.

NB that this is not a security issue: The only time this codepath is
called is in cases where either nestedp2m or altp2m is enabled, and
neither of them are in security support.

Reported-by: Matt Leinhos <matt@starlab.io>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Tested-by: Tamas K Lengyel <tamas@tklengyel.com>